I was tempted to offer a very high bounty to get a working xml-export. Alas, we already have https://gitlab.com/lilypond/lilypond/-/issues/665 and the there offered bounties are not that low ...
One thing I noted while skimming through that issue and the linked conversations: There were statements along the lines of "once we can use guile2, it gets easier" (because of the sxml module). Now we know since (I think) May that it is possible to produce a production-quality LilyPond using Guile 2.2. (I used Jonas' experimental binary release for quite some time over the summer in my everyday work, without any problems.)
So, while there are no regular official binary releases with Guile2 yet, I think it would now be quite acceptable to use Guile2 features in a possible XML export engine.
Lukas
